The present Invention generally relates to seals, typically dual seals, around movable elements such as moving, in particular reciprocating, elements in valves and pumps. More specifically it relates to apparatus and methods for detecting the failure of such seals.
In general, pumps have a variety of applications. For example, these pumps may be used in the oil and gas industry, coal and mining industry, and in sewage treatment plants. In the oil and gas industry, pumps may be, for example, reciprocating pumps. Reciprocating pumps may be used to pump fluids during drilling and completion operations and during various downhole operations such as fracturing, cementing, acidizing and water control. The pumps that perform these downhole completion operations may for example be transported to the well site via a truck or may be on a skid or an offshore platform.
Generally, for cementing operations, a wellbore is drilled and a casing is inserted into the wellbore. The casing is then secured to the wellbore with cement. Typically, a cement slurry is pumped down inside the casing to the bottom of the well and then up between the casing and the wellbore wall until the space between the wellbore and the casing is filled with cement. In hydraulic fracturing or fracture acidizing, fluids or slurries are pumped into wellbores in communication with subterranean formations at pressures and rates sufficiently high to fracture the formation. In acidizing, fluids are pumped into wellbores in communication with subterranean formations at pressures and rates sufficient to force the acid into the formation. In such operations the pumping pressures are typically very high, most typically thousands of pounds per square inch. Thus the fluid being pumped in these operations is at a pressure very much greater than the ambient pressure outside the pump and the flow lines. This is in distinction from such situations as sea-floor pipelines where the fluid being pumped may be at a pressure much lower than the ambient pressure, or various other situations where the pumps and the flow lines, and the fluids therein, may be at essentially the same pressure as the ambient external fluid, such as air at essentially atmospheric pressure.
Pumps that perform cementing, fracturing, acidizing and other operations have movable parts such as plungers and valves. These movable parts may utilize a positive displacement mechanism to move fluid (e.g., cement slurry or hydraulic fracturing slurry) from a reservoir (such as a tank) to the wellbore. The moveable parts are fitted with seals to isolate one chamber from another and prevent the undesired escape of fluid or pressure utilized for pumping. Pumps in such operations may typically reciprocate at 100 to 200 strokes per minute, which is therefore the frequency at which the valves operate, as opposed to valves that more typically operate at rates of from once or twice a minute to once a week or once a month.
If a seal fails, then the pump may also fail. Leakage of fluid past the failed seal may A damage the pump or the environment or cause the pump to stop operating. Thus, if a seal fails it may have to be replaced before the pump is functional again. Alternatively, another pump may have to be brought to the well site, or a standby pump previously brought for such an emergency may have to be used to replace the failed pump. In either case, downhole operations are delayed, the integrity of the operation may be compromised and the cost of the operation may substantially increase.
Detection of such leaks due to seal failure is extremely difficult; anticipation of seal failure is even more difficult. For example, U.S. Pat. No. 4,197,531 describes a method of first determining that an excessive amount of automatic replacement of a fluid between a pair of seals has been necessary and then sounding an alarm. In this apparatus, the pressure of the fluid between the seals is maintained at a pressure slightly higher than the pressure in the region into which the fluid might leak past a failed seal. Similarly, U.S. Pat. No. 5,746,435 describes a method for controlling leakage from a dual seal assembly by the sequence of steps of first placing a barrier fluid between the seals at a pressure above the pressure of the process fluid (for example, a fluid being pumped), second reducing the pressure of the barrier fluid, third detecting an increase in said pressure due to leakage, and finally replacing the removed or lost barrier fluid and calculating the leak rate. Finally, U.S. Pat. No. 5,772,216 describes a dual seal system for a valve in which a load member, that can add sealant from a supply, maintains a sealant above the pressure of both the fluid passing through the valve and the ambient pressure. Leakage of sealant past either seal is detected by monitoring either the volume or the pressure of the sealant supply. This is done either with a single device, internal to the valve, that measures the pressure differential between the sealant supply reservoir and the pressurized sealant between the seals, or by a mechanical device such as an indicator rod in the supply reservoir that shows a change in the sealant supply. In either case, these devices are complicated, specialized, integral parts of the valve or of the sealant supply assembly.
While the very high pressures at which fluids are being pumped in many oilfield operations are generally monitored, a decrease in the pumping pressure is not a good indicator of seal failure for two main reasons. First, initially the leak may cause only a small to drop in the measured pressure of the fluid being pumped; the change may be almost undetectable or may require much more accurate measurement than is usually employed. Second, some fluctuations in the pumping pressure could be normal; a drop in pumping pressure might not indicate a seal leak but might rather indicate only that the pressure in the location into which the fluid is being pumped has dropped for some reason. The typical pressure signal from the high-pressure side of a pump used at high pressures in the oilfield is very noisy and the fluctuations could amount to as much as plus or minus 20% or more.
Measurement only of the pressure on the other side of the seal, the low-pressure region in typical oilfield pumps, is also not a good way to detect seal leaks for several reasons. First, there is a tendency for the pressure on the low-pressure side of the seal to be lower when the high-pressure side pressure is low and higher when the high-pressure side pressure is higher. Consequently, if the pump is used in a low-pressure job and then subsequently in a high-pressure job, the pressure on the low-pressure side will be higher in the second job, but that might not indicate a leak. Second, breaking in of the seal, or the initial presence of foreign material in the low-pressure region might cause pressure fluctuations in the low-pressure region early in the life of the seal that are not really indications of a leak or imminent failure, and that will diminish after the first few jobs.
Thus, there is a continuing need for the simple advanced prediction of failure, or detection of the potential failure, of seals around movable parts in a conventional pump or valve or the like, that allows for replacement of the seals before leakage becomes excessive and performance deteriorates. There is a particular need for a system that does not require highly specialized valves or sealant supplies and does not require the sealant to be maintained above the pressure of both the fluid passing through the valve and the ambient pressure.
We have found that an improved method of detecting failure or imminent failure of seals is to measure the pressure on both the high-pressure and low-pressure sides and to compare either the ratio of the two or the difference between the two.
In one embodiment, an apparatus is provided for detecting pressure leakage across a seal comprising a first chamber disposed within and bounded by a housing, the first chamber containing a fluid at a first pressure that can be measured; a second chamber forming an annulus containing a lubricant at a second pressure that is less than the first pressure, the second chamber bounded by at least two seals disposed between the housing and a movable element, the movable element being disposed within the first and second chambers, the element adapted to move within the chambers, the seals sealing an outer surface of the moveable element, the seals sealing an inner surface of the housing, and the seals isolating the first chamber from the second chamber when the seals are working properly; and a sensor in fluid connection with the second chamber to measure the second pressure to enable detection of failure of at least one of the seals. Methods are also given for using the apparatus in valves, pumps and other devices to prevent damage and delay due to pressure leakage across a seal due to failure of the seal.
In another embodiment, a method is provided for detecting excessive pressure leakage across a seal due to failure of the seal comprising providing a first chamber disposed within and bounded by a housing, the first chamber containing a fluid at a first pressure that can be measured by a sensor in fluid connection with the first chamber; providing a second chamber containing a lubricant at a second pressure that is less than the first pressure, the second chamber bounded by at least two seals disposed between the housing and a movable element, the movable element being disposed within the first and second chambers, the element adapted to move within the chambers, the seals sealing an outer surface of the moveable element, the seals sealing an inner surface of the housing, and the seals isolating the first chamber from the second chamber when the seals are working properly; and measuring the second pressure with a sensor to enable detection of the start of failure of at least one of the seals.
In yet another embodiment, a method is provided for preventing excessive pressure leakage across a seal due to failure of the seal comprising providing a first chamber disposed within and bounded by a housing, the first chamber containing a fluid at a first pressure; providing a second chamber containing a lubricant at a second pressure that is less than the first pressure, the second chamber bounded by at least two seals disposed between the housing and a movable element, the movable element being disposed within the first and second chambers, the element adapted to move within the chambers, the seals sealing an outer surface of the moveable element, the seals sealing an inner surface of the housing, and the seals isolating the first chamber from the second chamber when the seals are working properly; measuring the second pressure with a sensor to enable detection of the start of failure of at least one of the seals; and replacing the failing seal.
The apparatus and methods of the Invention are used to prevent detrimental unplanned decreasing or ceasing of pumping in oilfield treatments and other operations. The apparatus and methods of the Invention are used to provide early detection or prediction of seal failure in order to allow performance of preventive maintenance and repair before catastrophic failure. This allows increased job reliability and less equipment, and possibly personnel.